Method of controlling the dimensional change when sintering an iron-based powder mixture

ABSTRACT

The invention concerns a method of controlling the dimensional change to a predetermined value including the steps of providing a first powder (A) consisting of an iron based powder ( 1 ) and copper in the form of elemental copper ( 2 ), or copper diffusion-bonded to the iron-based powder ( 3 ); providing a second powder (B) consisting of the iron-based powder ( 1 ) and a pre-alloyed iron-copper powder ( 4 ); mixing the first and second powder mixtures (A) and (B) in proportions resulting in the desired dimensional change adding graphite and lubricant and optionally hard phase materials and other alloying elements to the obtained mixture; compacting the obtained mixture; and sintering the compacted body.

The present invention concerns mixtures of iron-based powders.Particularly the invention concerns a method of controlling thedimensional change during sintering of compacts, which are prepared fromsuch mixtures.

Sintering of powder metallurgically prepared compacts based on iron oriron with alloying elements normally results in a dimensional change,i.e. the dimensions of the sintered product deviate from those of thecompact. The dimensional change is an obvious problem as various degreesof machining will then be necessary in order to obtain the identicalsintered parts required in mass production.

The variation in dimensional change during sintering is particularlypronounced when copper is included in the compact. Copper is widely usedas an alloying element due to its hardening effect. In contrast to mostother elements, copper causes swelling when it is included in the powderto be compacted. Dimensional variation or instability caused by swellingduring sintering of Fe—Cu and Fe—Cu—C-powder compacts have been studiedfor the past few decades. Different mechanisms have been suggested inorder to explain the swelling of the compact during the sintering. Thus,Bockstiegel, (Metallurgia, 1962, 3 (4), 67) proposed that the volumeincrease in Fe—Cu compacts during sintering was caused by solid statediffusion of Cu into the grains, leaving large pores at the originalcopper sites. Dautzenberg (Arch. Eisenhuttenwes., 1970, 41, 1005)conducted dilatometric studies and kinetic calculations and explained,on the basis of these studies, that diffusion alone could not beresponsible for the rapid volumetric growth during sintering. The rapidexpansion observed in the compacts was explained to be the effect ofpenetration of molten copper into the particle boundaries and along someof the grain boundaries inside the iron particles. The swelling effectof copper in different iron powders has been studied by severalresearches as for example Tabeshfar and Chadwick, (Powder Metall., 1984,27, 19-24), who showed that the internal porosity left in iron particlesafter compaction effected the degree of swelling.

Within the patent literature the dimensional change has been addressedto in e.g. the U.S. Pat. No. 5,567,890 which discloses an iron basedpowder including Ni, Mo and C for producing highly resistant componentswith small local variation in dimensional change. The dimensional changeof components produced by this powder composition is fairly independentof the sintered density and the carbon or molybdenum content. In theseiron-based compositions copper can only be present as an impurity. TheU.S. Pat. No. 5,507,853 suggests a method of improving the dimensionalstability of the iron-copper-carbon system by controlling the diffusionof graphite into the iron particle by adding selected oxides.

The Japanese patent application 53-146 204 describes aniron-copper-carbon sintered alloy, with good mechanical characteristicsand dimensional accuracy. The copper swelling is suppressed by addingthe copper as a prealloyed iron-copper powder.

In commercial powder metallurgical production dimensional change insintered iron-copper-carbon parts is generally controlled by addinggraphite to a combined carbon content of about 0.5 to about 0.8%.Addition of graphite to a iron-copper system has a diminishing effect onthe copper swelling and typically the growth can be kept below 0.4%. Bychanging the particle size of added graphite the dimensional change canfurther be controlled within certain limits.

There is however a need of controlling the dimensional change within awide limit without changing the chemical composition of the sinteredcompact and without adding large amounts of graphite or manipulatingwith the particle size of graphite. This is especially important whenthe same tool is used for plain iron-copper-carbon system as for highstrength material, such as iron-molybdenum-copper-carbon, which isdifficult to machine to right dimensions after sintering.

An object of the present invention is to provide a method of controllingthe dimensional change during sintering for systems including copper andoptionally also carbon and molybdenum. By the method according to theinvention the dimensional change during sintering may be controlled to apredetermined value without changing the chemical composition. Thepossibility of predicting the dimensional change will reduce the needfor machining and accordingly the cost of the final parts.

According to the invention the method of controlling the dimensionalchange to a predetermined value includes the steps of

-   -   providing a first powder (A) consisting of an iron based powder        (1) and copper in the form of elemental copper (2), or copper        diffusion-bonded to said iron-based powder (3);    -   providing a second powder (B) consisting of said iron-based        powder (1) and a pre-alloyed iron-copper powder (4);    -   mixing said first and second powder mixtures (A) and (B) in        proportions resulting in the desired dimensional change;    -   adding graphite and lubricant and optionally hard phase        materials and other alloying elements to the obtained mixture;    -   compacting the obtained mixture; and    -   sintering the compacted body.

The actual proportion can easily be determined by the man skilled in theart by small scale laboratory experiments or by using full scaleproduction equipment.

According to a preferred embodiment of the invention the iron-basedpowder (1) is an iron powder which is pre-alloyed with molybdenum.

In order to keep the same chemical composition for the mixtures and thesintered components produced from the mixtures with differentproportions of the first powder A and the second powder B, the coppercontent of the first powder shall be the same as the copper content ofthe second powder. This can be achieved either by adjusting the Cucontent of the powder A or adjusting the copper content of powder B. Thecopper content of powder B can be adjusted either by adjusting theproportions between powder (1) and powder (4) or adjusting the coppercontent of powder (4).

In order to obtain compacts having satisfactory mechanical propertiesaccording to the present invention it may be necessary to add minoramounts of graphite to the powder mixture to be compacted. Thus graphitein amounts between 0.1-1, preferably 0.2-1.0 and most preferably0.2-0.8% by weight of the total mixture to be compacted could be addedbefore the compaction.

The powder mixture is also preferably combined with a lubricant beforeit is transferred to the die. Examples of suitable lubricants are e.g.stearates, waxes, oligomers, polymers etc. The lubricants are preferablyadded in the form of particles but may also be bonded to the particles.According to the present invention the amount of lubricant added to theiron-based powder may vary between 0.05 and 1.5%, preferably between0.1-1.0% by weight of the mixture.

The compaction may be performed with standard equipment, at ambient orelevated temperature and the sintering may be performed at thetemperatures normally used within the PM field, e.g. at low temperaturesuch as 1100-1140° C. or higher temperatures such as 1250° C. and inconventionally used atmospheres.

An additional advantage by using the method of controlling thedimensional change according to the present invention is that annularcomposites consisting of one outer and one inner annular compact havingthe same chemical composition but different dimensional change may beproduced. This makes it possible to achieve a firm bonding between theinner compact and the outer compact.

EXAMPLE

Astaloy Mo, (available from Höganäs AB, Sweden) is a water atomised ironbased powder pre-alloyed with 1.5% of molybdenum. Astaloy Mo having 2%by weight of copper diffusion bonded is also available from Höganäs ABas Distaloy DH-1. Distaloy DH-1 is in the following referred to aspowder A.

Astaloy Mo mixed with 10% of Astaloy 20 Cu, which is a water atomisediron powder prealloyed with 20% of copper and which is also availablefrom Höganäs AB) was used as powder B.

Ten mixes were prepared with different proportions of powder A andpowder B and different graphite contents. 0.6% of Kenolube™ lubricantwas added to all the mixes. The following mixes were made:

Proportion of proportion of Mix no powder A % powder B % graphite % 1100 0 0.4 3 70 30 0.4 5 50 50 0.4 7 30 70 0.4 9 0 100 0.4 2 100 0 0.6 470 30 0.6 6 50 50 0.6 8 30 70 0.6 10 0 100 0.6After mixing and addition of lubricant fourteen tensile test samples foreach mix were moulded, with a mould pressure of 600 MPa in a uniaxialpress movement. Seven of the produced tensile test samples of each mixwere then sintered at 1120° C., for 30 minutes, in an atmosphere of 90%N₂/10% H₂ with a carbon potential of 0.2% and the rest of the tensiletest samples were sintered in endogas atmosphere, at 1120° C., for 30minutes, with a carbon potential of 0.5%.

Dimensional change and mechanical properties of the samples weremeasured and the mean value based on seven samples treated during thesame process conditions, were calculated.

FIG. 1 shows the value of dimensional change from samples produced frommixes with different proportions between powder A and powder B. Thefigure shows that a very accurate predetermined value, in the range from+0.2% to −0.14%, of the dimensional change can be reached by changingthe proportions of powder A and powder B.

FIG. 2 shows the mean value of the sintered density, FIG. 3 shows themean value of the ultimate tensile strength, FIG. 4 shows the mean valueof elongation and FIG. 5 shows the mean value of the hardness for sevensamples produced from the same mix and sintered under the sameconditions. The figures show that the variations in sintered density,tensile strength, elongation and hardness are very small and withintolerable limits.

1. A method of controlling the dimensional change to a predetermined value when sintering a compacted body of a powder mixture comprising: providing a first powder (A) consisting of an iron based powder (1) and copper in the form of elemental copper (2), or copper diffusion-bonded to said iron-based powder (3); providing a second powder (B) consisting of said iron-based powder (1) and a pre-alloyed iron-copper powder (4); wherein the copper content of powder B is made equal to the copper content of powder A either by adjusting the Cu content of the powder A or adjusting the copper content of powder B, whereby the copper content of powder B is adjusted either by adjusting the proportions between powder (1) and powder (4) or by adjusting the copper content of powder (4); mixing said first and second powder mixtures (A) and (B) in proportions resulting in the desired dimensional change, adding graphite and lubricant and optionally hard phase materials and other alloying elements to the obtained mixture; compacting the obtained mixture to a compacted body; and sintering the compacted body.
 2. The method according to claim 1 wherein the iron based powder (1) is an iron-molybdenum pre-alloyed powder. 